Merge with nested dicts
Input
deep_merge({'x': 1, 'y': {'a': 10}}, {'y': {'b': 20}, 'z': 3})
Output
{'x': 1, 'y': {'a': 10, 'b': 20}, 'z': 3}
y is merged recursively; x and z are preserved/added.
Full lesson preview
Deep merge nested dictionaries
Merge two dictionaries recursively, combining nested mappings instead of overwriting them.
Python practice25 minList & Dictionary PatternsAdvancedLast updated March 20, 2026
Problem statement
Implement deep_merge(dict_a, dict_b) that returns a new dictionary representing dict_a merged with dict_b. Rules: (1) Keys present only in one dict are included as-is. (2) If a key exists in both and both values are dictionaries, recursively deep-merge them. (3) If a key exists in both and at least one value is not a dictionary, the value from dict_b overwrites the one from dict_a. Do not mutate the input dictionaries; always return a new dictionary.
Task
Write a deep_merge function that merges two dictionaries recursively: when a key exists in both and both values are dicts, merge them; otherwise the second dict's value overwrites the first's.
Examples
Input format
Two dictionaries, dict_a and dict_b.
Output format
A new dictionary representing the deep merge of dict_a and dict_b.
Constraints
Dictionaries may contain nested dictionaries. Other value types should be treated as atomic and overwritten by dict_b when keys collide.
Samples
Sample 1
Input
deep_merge({'a': {'b': 1}}, {'a': {'c': 2}})
Output
{'a': {'b': 1, 'c': 2}}
Nested keys under 'a' are merged.